Have you really read the post above yours ? I mean the left side piece of info... Do you think he didn't understand your question? He just gave you the bottom line...
Maybe you didn't mean to sound sarcastic, in this case forget about my reaction.

I answered your question perfectly. It is not possible to install the iPad baseband on a iPhone 4/4S meaning no one has tried it. If anyone has tried they either failed or bricked their phone. Anyone with any understanding of the iPhone 4 baseband would know not even to try.

Would you like to know the "why" it is not possible and no one has tried it?

The reason that you can't upgrade iPhone 4/4s to iPad baseband 6.15 is that Apple uses different chipset, so they are not compatible. With the later built 3gs, Apple made a change to Toshiba chip. Thus, when people tried to flash the Toshiba chip with iPad baseband, it cleared the chip and rendered the phone useless i.e. an iPod

Just to add to that even if the baseband was compatible with the iPhone 4 there would still be no way to install it since iPhone 4 has baseband SHSH (nonce ones, which is why they cannot be saved like regular pre 5.X SHSH). So you can only change the baseband on a iPhone 4 when Apple supplies those baseband SHSH. Which they obviously never would for 6.15.